In this paper, I examine Scotus’s claim that the categories are the subject of a propter quid science. In order to see the significance of this claim, I first trace the development of the idea that the categories are the subject of a science from Martin of Denmark, Peter of Auvergne, and Simon of Faversham. I then turn toDuns Scotus’s account of the categories as the subject of a propter quid science. Throughout the discussion, I concentrate on the fundamental problems confronting anyone who claims that there is a science of the categories: namely, how they, being ten, can have an appropriate unity. Scotus, as we will see, will answer this problem by claiming that the intellect causes a greater unity in second intentions than the corresponding unity that exists in the world. As a consequence, Scotus contends that the categories are the subject of a propter quid science, one that is radically different from the science of metaphysics